The earlier suggestions about there being upgrades about each five levels or so is assuming standard gear - the sort that can be crafted (normal quality) or bought from NPC merchants. If you get a dungeon drop, that's going to be better and may exceed the stats on standard gear that's several levels higher, so it won't need to be replaced as soon. High quality crafted gear also tends to be roughly on par with the following tier of normal quality gear.

The market board in this game has a nice feature in the fact that it shows all tradeable items regardless of whether anyone has it up for sale at the moment or not. I've found it useful to periodically take a look there just to see what exists for my level (or the next level or two up). If someone has it for sale cheap, I might buy it there, but if not at least I know what gear to look for or to try making (if I'm high enough in the corresponding crafting class).

Another really good source for finding what gear is good for a given level is at the Crafting as a Service website. Click "gear me out", select your class and level, and choose whether or not you want craftable results only. It will then tell you what the best gear for your level is as well as when a better piece will become available for the next several levels. It's a really well done website, really useful for crafters, but this section is useful even for non-crafters wanting to know how their leveling gear stacks up.